I have the 12.5" Progressive 440's. They are one of the best bargains going in my book. I wish I got them a little taller to give me some more conering clearance though. Who knows, maybe I will sell mine on ebay one day and get the 13 or 13.5" shocks.
If you don't want to spend the 350-400 bucks for the 440's Progressive has other models that Rocket riders have used and enjoy. Of course if you want a better suspension you can check out Wilbers, WP, maybe Ohlins, I suppose the sky is the limit. Whatever fits the VTX1800 should fit our bikes so you can check out other manufacturers if you want to shop around for other brands.